X-rays, gamma rays, and particle radiation are currently used in many types of analytic instruments. By using radiation, much can be learned about the composition, structure, and other characteristics of a sample. Unfortunately, conventional instruments have limited intensity and/or limited control over beam direction or divergence.
One of the most important and widely used nondestructive evaluation methods for a sample (materials, components or systems) is X-ray fluorescence analysis or spectrometry (XRF). This technique uses energetic photons (X-rays) to induce excited electronic states in the atoms of the sample being studied. The atoms are then de-excited by emission of X-rays with an energy equal to the energy difference between the excited state and the ground state of the atom. Such emissions are characteristic of each element. By measuring the spectrum of these secondary X-rays, a quantitative measure of the relative abundance of each elemental species present in the sample can be obtained. This technique is typically fast, nondestructive, quantitative and very sensitive (in many cases parts per million can be detected) and is widely used for separated sample laboratory analysis and real time analysis during manufacture, processing and testing. Because of its quantitative nature and low vulnerability to matrix effects, it is frequently used to calibrate other analytical techniques.
X-ray fluorescence spectrometry, as an analytical tool, has developed primarily along two paths: wavelength dispersive spectrometry (WDXRF) and energy dispersive spectrometry (EDXRF).
Wavelength dispersive fluorescence spectrometry (WDXRF) is older and employs crystal diffraction of X-rays to measure wavelength. Since Bragg diffraction of X-rays is very precise, the wavelength resolution is typically very good, resulting in high sensitivity and, in some cases, determination of the chemical state as well as the amount of sample constituents. Measurements are carried out by using a goniometer which changes the angle of collimated secondary X-rays relative to the planes of an analyzer crystal and a detector which measures the intensity of defracted X-rays. Because the angular requirements have very tight tolerances, some systems employ two collimators, a primary collimator between the sample and the crystal and a secondary collimator between the crystal and the detector. Although collimation increases resolution, it decreases sensitivity because photons are absorbed by the collimator. Some systems are configured to allow the operator a choice of a range of fine to coarse collimators to adjust the tradeoff between resolution and sensitivity. The strict angular requirements and sequential nature of such a system frequently results in measurement times of minutes or hours to obtain a complete spectrum of wavelengths and therefore composition of the sample. However, if the amount of a particular impurity or constituent is of interest, the goniometer can be set at the proper position and the measurement can be made in a shorter time. Alternatively, a number of monochromators, each containing a crystal and detector set to measure wavelength for a particular element can be used to simultaneously determine the relative amount of a number of elements. Furthermore, crystals can be bent to allow a range of incident angles to be diffracted, thereby relaxing the collimation conditions. This is frequently done for process control measurements.
Energy dispersive fluorescence analysis (EDXRF) measures the energy spectrum of secondary X-rays, typically with a semiconductor silicon or germanium detector. An incident X-ray photon stops in the detector by exciting electrons from the conduction to the valence band of the semiconductor. The resulting electron-hole pairs are swept apart in the electric field applied across the semiconductor diode and the number is proportional to the photon energy. By sorting the charge pulses in a multichannel analyzer, the entire energy spectrum of incident X-rays can be determined. This detection method has inherently poorer resolution (determined by the electronic resolution of the detector-electronic system and ultimately by the statistics of the photon slowing down process), and is frequently limited by the high counting rate in the detector of photons other than those of interest. In spite of these limitations, simplicity, lower cost, and the convenience of getting the entire spectrum at one time, frequently makes EDXRF the technique of choice. The two described measurement techniques may be combined to get rapid, semi-quantitative results followed by high resolution measurements, although few commercial instruments can accomplish this.
X-rays produced by photon bombardment of solid targets include monoenergetic X-rays characteristic of the target material on a broad background of "bremstrahlung" radiation. Secondary X-ray spectra excited by such an X-ray source usually has a background especially at lower energies from scattering of the bremstrahlung continuum radiation in the sample. Characteristic X-ray production from a particular element is most efficient when the exciting X-rays are just above the absorption edge energies of the element of interest. Both background reduction and increased efficiency requirements can be met by using a "secondary target excitation" approach in which the primary X-rays are incident on a target composed of selected element(s) to give nonenergetic X-rays of appropriate energy for excitation of the sample. These techniques together with the use of selected filters between the primary and secondary source is called Source Tuned X-Ray Fluorescence, or STXRF. Because the X-ray intensity is very much reduced from a secondary target, this technique has been used principally with EDXRF, although the same benefits should be realized in WDXRF measurements.
The subject invention provides a solution to the long felt need in the art for improved X-ray fluorescence analysis by (1) decreasing measurement time due to increased intensity reaching the detector, (2) decreasing measurement time due to decrease in bremstrahlung radiation reaching the detector causing photon pile-up, (3) increasing resolution by increasing signal to noise ratio, (4) decreasing the bremstrahlung radiation reaching the detector, (5) increasing resolution for WDXRF by very precise control of angular relationships, (6) improving ability to evaluate small well defined areas, (7) improving ability to determine the distribution of constituents by scanning, (8) improving ability to determine average composition over a well defined area without moving the sample, source aperture, or any other part, (9) allowing analysis of well defined interior volumes in a solid, and (10) reducing the cost of other components in system, such as by allowing the use of smaller detectors.
X-rays optics encounter difficulties different from those in the visible and IR ranges. These difficulties stem from the fact that the surfaces of all known materials have very low reflection coefficients for radiation at large angles of incidence. One way to overcome this problem is by using grazing incidence to take advantage of the total external reflection of X-rays. This is done in X-ray telescopes and microscopes and in mirrors used in synchrotrons for deflection and focusing. Such applications operate on the basis of one or two reflections and have an extremely small angular aperture because of the small value of the total-external-reflection angle (about 10.sup.-3 radians). On the other hand, diffraction and interference elements such as Bragg and multiple layer mirrors, zone and phase plates, and gratings are very wave length selective and therefore cannot be used to control X-ray beams having a wide energy distribution.
The inventor of the subject invention first proposed focusing X-rays by multiple reflections from surfaces with certain special shapes and carried out systematic investigations of this suggestion demonstrating that transmission through these "Kumakhov" lenses could be as high as 50%. Moreover, even with lower transmission, an increase in X-ray intensity (as great as four orders of magnitude) is obtained due to the large collection angular aperture possible (0.25 rad).